Resolve "Uploaded project image not validated"

Acceptance criteria

• Coverage is higher or equal to coverage on master
• CI runs without problems
• Linter does not contain any additional issues
• Code doesn't contain TODOs, unused functions or console outputs
• Code has been inspected and deemed to have decent quality
• Code does contain license headers
• If necessary, the changes are documented in the changelog
• If this is a bug fix, a test covering the bug is present [effectiveGo]: https://golang.org/doc/effective_go.html

Closes #305 (closed)