1. Download steamcmd.exe https://developer.valvesoftware.com/wiki/SteamCMD <<<< The ONLY legit steamcmd LINK. Do not download any other.
2. Upload your addon.
3. Copy the addon ID (Copy page url of your workshop item, the Addon ID will be the numbers at the end of the link)
4. Navigate to your steamcmd install directory.
5. Create a text document inside the directory and put following lines in.

"workshopitem"
{
"appid" "1118200"
"previewfile" "Put the file path to the gif here, simply hold shift and right click your gif in your file explorer and press Copy Path"
"publishedfileid" "ADDONID (the numbers at the end of the workshop url)"
}

fill in the fields accordingly and name it gifaddon.vdf

6. Execute steamcmd.exe and wait for it to finish loading.
7. Enter "login" and them type your email and password, don't worry, it's safe.
8. Type "workshop_build_item gifaddon.vdf"
9. Wait for it to finish uploading the new thumbnail.
10. Then type "logout" and then "quit".
11. Relaunch steam and view your addon.

Notes:
-you will need to login again when relaunching steam.
-When you enter your login, it might ask you for your mobile authentificator code, so enter that when it asks for it.
-Steam becomes unresponsive after the steamcmd login, thus make sure your addon is already uploaded, when you're done, u have to relaunch steam.