For the item in Sitecore 'Media Library':
MediaItem mediaItem = Sitecore.Context.Database.GetItem("{E47591D0-48D2-4543-80E4-3836B02AA1A8}");
The above item is a picture "Tulips.jpg".
How can I check, from code behind, that the above item is actually a Picture/Image?
Once you have your MediaItem
object, you can do this:
MediaItem mediaItem = ...
bool isPicture = mediaItem.MimeType.StartsWith("image/");
This works because MIME types of images are formed like this: image/...
. For example: