To win a photo usually which is better looking than mediocre. It often implies that the one who says "사진 하나 건졌다" is fairly satisfied with the picture she/he takes because it's a little unexpectedly better looking or extraordinary than the most of pictures the one gets routinely. Girls usually say that when they're taking a selfie of themselves. like "Yay!! I got a nice pic of myself looking a ton better than I am actually!!" or people taking good scenic pictures on landscapes during a travel or so.

건지다 means " fish (sth) out (of), scoop" 사진 하나 건졌다 literally means "I fished one picture out of many pictures" It means "I got to have a nice picture"