http://modxcms.com/forums/index.php/topic,54972.0.html

Template Variable

Name: e2g-galleries
Caption: e2g-galleries
Description: Image gallery selection
Input Type: Listbox (Multi-Select) (I still have no idea how to make the box gets longer, since MODx looks like limits the height)
Input Option Values: @EVAL return $modx->runSnippet('getE2GTree');
Widget: Delimited List
Delimiter: , (comma)

Don't forget to apply this TV to your Template in the Template Access check box.

Snippets

We need 2 snippets:
1. 'getE2GTree' : getE2GTree.txt
Description: The image gallery tree inside the 'e2g-galleries' Template Variable.

2. 'getE2Gids' : getE2Gids.txt
Description: TV Parser for Easy 2 Gallery's front-end snippet

Snippet Call

[!easy2? &gid=`[[getE2Gids? &give=`gid`]]` &fid=`[[getE2Gids? &give=`fid`]]`!]

Please provide both parameters, since the TV also feeds both.
Please notice the cached and uncached call.