This is a short Bash script that uses cURL to send a profile change request to Steam's servers.
No bugs or exploits have been used in the making of this script, as it does the same thing your Steam client or web browser would do.
This script currently only runs on Linux, however there are alternatives available for Windows. If anyone would like to port this script to Windows, feel free to do so.

While using this script, keep in mind that Steam might change the way it handles requests and, as a consequence, this script might no longer work as intended.

This script was written entirely in Unix Bash, and requires no additional libraries.

• A working Linux installation. I used Ubuntu 15.04 vivid for developing, testing and debugging.
• Some experience with websites and script debugging might help.

These are important to have, as the script does not implement logging into Steam on its own, and relies on pre-existing session cookies to be provided. Don't forget that cookies might expire and become invalid after a while, so you will have to replace them with the new ones.

There are several ways of obtaining Steam cookies. I will be focusing on using Firefox, as I used this browser during testing.
For both of my described methods, you will first have to successfully log into steam in Firefox.

The first method is to export cookies from Firefox using the Cookie Quick Manager addon. Click on the addon icon, go to 'Manage all Cookies' and search for 'steamcommunity':

Find the following lines:

sessionid: eigiidremp266aivzm18ms1r
steamLoginSecure: 76561198053562138%7C%7CdxkaMINHZGXDzFrvrRKgfHmhDAcjWNqfAYNzUQHJtFuMvfiYXRsmQFO...

• The first line is your sessionid cookie.
• The second line is a combination of your steam64ID and steamLoginSecure cookie. It is combined in the following way:

{steamID64}%7C%7C{steamLoginSecure cookie}

Note these down somewhere for when setting up the config.cfg file.

The second method is to look at the cookies used in page requests, when browsing steamcommunity.com. In Firefox, this can be done using Firebug.
After installing and launching Firebug, go to any page on steamcommunity.com (for example, your Steam profile page).
In Firebug, go to the Net tab, and All section. Find the GET /id/yourID/ request (or similar), and look at the Cookies section of the request. For convenience, this is illustrated in a picture below.

Find the appropriate cookie values and treat them like in the first method (except %7C becomes |)

Again, note these down somewhere for when setting up the config.cfg file.

By default, the script expects itself to be in the following directory:

/home/user/Documents/SteamAvatarBot

This is where the script expects the config.cfg file to be in and was done to allow the script to be automatable.
To have it run somewhere else, you will have to change the $scriptFolder variable (line 20 in change.sh).

The config.cfg file consists of a total of 7 lines.

• Line 1 - The location of the picture folder. By default it is set to the following directory

/home/User/Pictures/SteamAvatarBot

You may leave this as it is, or change it to something else.

• Line 2 - Set this to the total amount of avatars in the folder. The script currently does not implement folder browsing, thus iterating the filename is used.
• Line 3 - The current avatar that is used. Set this to 1 when setting up.
• Line 4 - Your sessionid cookie.
• Line 5 - Your steamID64.
• Line 6 - Your steamLoginSecure cookie. Make sure the values are on the exact lines specified, with no extra whitespace.

For now, the script uses a very rough way of file handling.
All of the pictures must be numbered from 1 to n (total amount of pictures), and must be in .png file format. If you want to use another file format, modify the script on line 56.

There are two ways of using this script.

One way is to manually launch it every time you want a new profile picture. This can be done by executing:

bash <script path>/change.sh

For now, the CLI is very basic, and does not offer much debug ability. However, the script can be modified to do so.
You may add the following command to the script:

echo $output

This will output the response from the POST request, and can provide insight to errors.
Also, -s can be removed from the curl command to see all of the request data.

For automating the profile picture change, crontab can be used. Sample crontab command to run the script every 30 minutes:

*/30 * * * * bash ~/Documents/SteamAvatarBot/change.sh

After further testing of v1.0 of the script, the following issues have appeared:

• Running the script on a Raspberry Pi requires some variable cleaning (removing \t from every line read from the config.cfg file).
• Steam cookies might expire and cause the POST requests to fail.
  I will try adressing these in further releases.

This is one of my first Bash scripts ever written, so do not hold any mistakes against me.
I am not sure about my stance on providing support. You can certainly try contacting me for support, but do not expect me to fix every issue you might have. I managed to get this working from scratch in around 5 hours, so you should too.

If you do have any questions, you can contact me on Steam.

• Stackoverflow, for being Stackoverflow.
• Leystryku, whose code I found near the end of development. Helped narrow down some issues I was having.