Here is a wide collection of nice-looking professional photos from around the internet. These are styles that read well as "highly educated, motivated, experienced, trustworthy." You can click on an image to expand it.
Full disclosure: these are not my photos, I Googled them.
- For men: you'll see that the looks are mostly with a jacket, some with tie, some without, and all with neat, "not floppy" hair. Have them bring more than one jacket if they aren't certain about fit or style.
- For ladies: the standard appears to be jacket and blouse and a simple (no more than two strands) necklace. Hair is generally down, but out of the face. If uncertain about blouse style, bring two options.
Also, I made some notes about each of these images from a "how would one describe these" perspective on the following pin board: