issue #57: Implement image validation in backend

[MaxenceGui]

Tasks

• [x] Add an image validation function in the backend

• [x] Implement extension validation check

• [x] Implement resizable validation check

• [x] Implement size validation check

• [x] Implement file header validation check

• 57

[MaxenceGui]

Just optional considerations if even relevent.

Your comments are always relevant and welcome

[rngadam]

revisit environment variables to set reasonable defaults when a variable is not set.

[MaxenceGui]

revisit environment variables to set reasonable defaults when a variable is not set.

Done

[MaxenceGui]

Needs to be rebased

Rebase done