Our Product is Photopost Pro not PhotoBlog as that is some vb
hack I beleive if I remember correctly. There are 6 standard extra fields that are definable in each category that are searchable. Additional extra fields are not searchable. So if you wanted to define normal extra fields and edit the templates to change say a normal input box for the regular extra fields to a dropdown you could do what you want.
The reason most galleries that have additional extra fields are not searchable is it is near impossible to do so. They are stored as an array and in different tables in an array and that array is not really data it is numbers etc. You then need to grab against another table to grab the real data. The way a search is done it searches a specific field for the search string. In the arrays you need to compare one table to another table just to grab any data and convert it to something readable as we print the html to show what is stored so there is no reliable way to search things. I know I tried looking at it awhile ago but could not find a fast way. All queries where unacceptable to try and get results in a timely manner.