Currently, I'm not inclined to add this option...mostly because I can't imagine people not wanting to have a fair amount of control to what the text would be, how it would be styled, and where it would be displayed in relation to the picture. That'd be a lot to take into account.
Also, it would probably have to have some type of size restriction...so small thumbs didn't get text, but larger ones did.
For now, I'd recommend the Advanced PHP
solution for this type of customization. It isn't really as \'advanced\' as you may be thinking. In addition, I would be happy to assist if you choose to give it a shot and have the ability to add PHP on your page.
If you do not, feel free to detail your request -- even make a mockup or something...I may talk tough, but in the long run I just want to make people happy. If I'm not working on something else urgent, I'll probably do my best to help out.