too_many_images
Status 409. The store already keeps as many pictures as it may.
What it means
A store keeps at most 20 pictures. A new one normally takes the place of the oldest the design does not use; this answer means the design uses every one.
What to do
Point the design's brand at fewer pictures with POST /stores/{store_key}/design, then upload again.
Safe to send again?
Not as it is. Change what the answer points to, then send the request.
Where you can meet it
POST /stores/{store_key}/images: Upload a picture for the store's branding (a logo, a square logo, a cover)
Every code, and the shape of an error answer: Errors.