Not rated
There are a few comments on top of the file, description of the snippet, and how to load.

    (3/5)
This snippet does exactly as the author explains, it allows you to view pictures in mIRC.
 
The snippet uses dialogs and custom windows. Using mIRC builtin identifiers to specify a directory and load all picture "JPG format" file into a list, and on clicking a picture name it shows a small thumbnail in the dialog. Then when you click "Load" it opens a custom window showing the picture in its actual size.
 
The "Picture viewer" dialog which allows you to specify a directory has an option to open a single picture file in all formats, not only JPG files, although mIRC does accept other picture file formats as a thumbnail in the dialog icon control.
 
The could be better if it had the options to send the picture via DCC or set it as a background, or even copying it to the mIRC directory, delete picture, etc...
 
The dialogs look nice, noted few bugs mentioned in the Overall section.

     (4/5)
The snippet contains dialogs, and the dialogs looks pretty nice, not much wasted space, no excessive pointless size and decent alignment.
 
Could be improved a bit by adding more options, like setting a default directory, caching the last opened directory, opening a fullscreen preview.

This snippet is not really original, but it is layed out decently. Despite two bugs found it does work great.
 
Bugs:-
1] The custom window is smaller than the picture actual w,x,y,z sometimes, especially with small pictures.
2] The 'Load' button is always enabled when there are not pictures loaded, which brings an error "* /drawpic: insufficient parameters" when clicked.
 
ExOrCiSt: Your coding is getting better. Please pay attention to the suggestions :)