Screen or print or both?
 

Successful gallery images appear to need a combination of (a) appeal at thumbnail size (b) the appeal satisfied at monitor size in terms of both content and technical competence (c) something with a new twist or worth debating.

A pic can do all that yet fall totally flat if printed (assuming good print quality) and even if an interesting print to pore over for a few minutes (eg as a greetings card), still won't necessarily hang gracefully on a wall.

Anyone any thoughts on whether most images have to be dedicated to a specific showcase, or has anyone found a universal answer? Do those of you that post with electronic frames find such images are more successful printed and if so in a similar frame or what?

A simple graphic composition will work in all formats, but really as a work of design rather than detail. (The 'less is more' philosophy of landscape composition for example) Detailed images work best as 'in the hand' formats - cards or small-medium prints (eg up to A4), or as jigsaws!
For a thumbnail, either very strong composition or a striking subject larg ein the frame are needed - there is not enough space for subtlety.

When posting on the internet I generally put a key line and white border round the picture to isolate it from the screen.

For prints I put a key line round the image and then mount, leaving a 3/4" white border, and then frame.

I think that images which dont work as prints, look OK on screen becuase you dont spend as long looking at them.