How can I apply diamond dust to a canvas or photo? I've searched the web but can't find any information about it. I've used photo spray glue but it's too sticky.
"Diamond dust" refers to glitter than is actually composed of crushed glass instead of the traditional shiny flecks of plastic. This differs from glitter in that it is composed of larger pieces and therefore has more problems with adhesion. My guess, based on the limited information, is that Summer Rose is having a hard time because the glitter is leaving gaps where the adhesion shows through and is too sticky? To avoid this, there are a couple options.
1. Use a better glue
For glitter like this that is both heavier and larger, a more robust glue is needed. Instead of spray glue, try using PVA glue or hot glue. If you find these adhesives do not dry completely clear and do not offer the desired effect, there is diamond dust specific glue called Diamond Glaze which seems to be popular among crafters if you'll be using diamond dust frequently.
2. Use traditional glitter instead
Traditional glitter is cheap and super easy to use. It can be adhered with nearly any liquid glue. Just don't accidentally drop it on yourself.
This forum on diamond dust also might be helpful to you where a similar question is asked. It's lengthy, but you might get some good tips outs of it!
Make your own. I buy my seed beads at the dollar store: sometimes Walmart or the craft stores when on sale. Use a coffee grinder, different small shifters. Wait till the dust settles in grinder (you do not want to breath any particles), then shift with smallest shifter 1st, then 2nd and so on. You'll get 3 to 4 different grits. To apply use triple thick, or beacon glue product (they are very good). You may want to buy the christmas glass balls. Put them in 2-3 strong bags, secure and bang them to pieces: use a rolling pin to crush then to bits. If you like use grinder 1 to 2 pulse--shift, then grind the larger pieces. I tell you they look like german glass glitter. So save some money and DIY. Happy grinding.