Star Army allows you to upload art that you have the rights to use. This could be art you made, art you bought, or art you've licensed. One type of art that is permitted is stock imagery, which can be used to illustrate wiki pages (such as an RP setting) or as resources for making new images.
Stock images must meet the following criteria to be used on Star Army: they must be commercially-usable and royalty-free, public domain, or licensed under creative commons or similar copyleft; or they can be copyrighted images that the owner has given you permanent permission to use. They should not violate the Site Terms of Service. In general, the less restrictions, the better.
Keep in mind that stock photography only has limited use, as pictures will be of things in the real world and often contains things that don't exist in Star Army. Don't use stock that has recognizable Earth brand names, trademarks, or landmarks (such as the Eiffel tower, etc). This is especially true for images of cars and technology.
Unmodified art from from outside sources needs to be properly noted. On the wiki, there are special media namespaces for these:
Each of these media folders have subfolders that identify the licensing and/or source. Note that use of unmodified stock can be considered “editorial” use and may be subject to restrictions. Never upload large unmodified original stock files to the wiki!
Above is an example of a media file on Star Army licensed under Creative Commons CC0.