Would "beach" and "desert" be appropriate keywords for a photo with sand?

Desert islands are both.

Southern California is a coastal desert. 

If you look hard enough, you'll find seashell fossils inside most desert rocks.

Use both if it fits your asset. 

Hello,

The key point to remember is that the keywords used must be indicative of the type of picture/asset it relates to!

If it's not obvious if it's a beach or a desert picture, you can use both keywords. 

Are there any beaches in a desert? Seems likes it's one or the other...

I can't imagine that being a problem. Keyword spamming to me would be using words entirely unrelated to the subject.