This is my stance:
If a picture is posted or tweeted on social media, which says on it the creators name/web address/copyright info or similar, share it all you like.
However, if you want to use an image on your website, or as a header on a social media group or campaign or blog or anywhere else where it is fixed and linked to you/your organisation/charity/social media group, then ASK. If you want to print it, ASK.
This is important.
With email and social networking sites asking is very easy and need only take a few moments. Then you will get an answer. It may be yes, it may be no, it may be 'if you pay for it'. But you will not be taking away my livelihood.
If you ask, you are much more likely to get a yes than if I discover you are using an image without permission. In which case expect to be told to remove it - even if I would have said yes had you asked, if you cannot show me the courtesy of respecting my work, why should I give you permission to use it?
If you are planning to use it for a good cause, or as a charity, or for awareness raising, this DOES NOT give you a right to use an image without permission. Seeing as virtually all my work promotes understanding and acceptance of disability, this theory means I have no right to earn a living. My images are how I earn money to live. It is applicable to the work of thousands of charities and non-profit organisations and groups across the globe. If every good cause used my images without consent my images would be everywhere but I would a) not earn enough to feed myself and b) have to close my business.
And I apply the same principles to the work of any other artist too.
If you want to support artists, then, quite simply, ask for permission before you use an image.