First, to be clear, I have not done this and will not do it if it’s not acceptable. But I would like to know whether, when someone has shared an image file on this forum with a sharing license, is it ok to share their image on other social sites as long as I attribute it to them and explicitly state the same license?
Short answer: Yes that is OK.
Do have a good look at the license though!
Most licenses state what it is you can (not) do with them. If you are unsure and it is a Creative Commons (CC) license you can always look it up: About CC Licenses
For example, I license my images this way:
This image is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License (CC BY-NC-SA 4.0)
@Jade_NL already answered and I’m just echoing his sentiment.
If possible it would also be awesome to share a link to the original post that had the image.
All of the licenses we ask people to explicitly state here generally have at least a BY clause in them (for attribution).
Some have the SA clause to Share Alike - meaning derivatives are under the same license.
Some also have the NC clause: Non-Commercial - meaning no commercial use (the exact meaning of that is still sort of nebulous, which is why Wikipedia won’t allow it as an acceptable license).
There’s a short breakdown available on their site (actually, the link from @Jade_NL is a little more succinct):