I think it also depends quite a bit on the "application culture" of the country where you're applying for work. In Germany, it's customary to send a whole folder of evidence (CV, certificates, reference letters...) along with your application, at least for permanent jobs. As such, I'm not too worried about sending scans of my certificates to respectable-looking companies or agencies--presumably they've got plenty already and wouldn't need mine for fraudulent purposes!
I like José's suggestions for other countries or clients you're not sure about, though, especially regarding referring people to online directories confirming qualifications. You could also watermark the PDFs of your certificates if you have Adobe Acrobat, or if you're willing to use an online PDF editor.
José Henrique Lamensdorf wrote:
... My sworn translator diploma was badly scanned from the original. Nevertheless, I tell them that it's worth nothing on its own. If they want to check it, I give them the URL where my name must be in the official directory, to evidence that I am STILL licensed as such.
In both cases, though the diplomas are easily readable, not even the best Photoshop artist would be able to rebuild these scans to look like originals, much less to replace my name there with any other without making it stand out like a sore thumb.
If their intentions are good, they can see and read these diplomas. Otherwise, they'll delete them pronto.