Check image size of #BACKGROUND and #COVER

[MacLemon]

Add a check for images so they comply with a minimum quality size requirement.

General requirements and aspect ratio.

´#COVER´ should be mostly square ±10% and an even resolution.
´#BACKGROUND´ aspect ratio should be 16:9

Requirements for Full-HD display:

´#COVER´ should be at least 300px × 300px.
´#BACKGROUND´ should be at least 1920px × 1080px.

Requirements for 4K display or Hi-DPI.

´#COVER´ should be at least 600px × 600px.
´#BACKGROUND´ should be at least 3840px × 2160px.

[luto]

soooo... should I check for 300x300 or 600x600? ;)