If you are having problems with changing your wallpaper, then try to reset Symbaloo's system.
To reset your backgrounds perform these steps:
1) Press F12 for Windows/Command + Option + I while on your Symbaloo page. The Chrome Developer Tools will pop up from the bottom. Press the 'gear' symbol 2) Click the Settings Gear (image) in the top right of the Developer Tools. 3) Look to the left and find General and click 'Disable Cache'. 4) Don't leave the tools, just reload the Symbaloo page. 5) Re-check enable cache. 6) Exit the Developer Tools 7) Change your wallpaper