How can I show all images in a gallery, except the first?

To show a subset of the images in a gallery you can use the &offset and &limit properties on the mgGetImages snippet. Your snippet call might look like the following:

[[!mgGetImages? &offset=`1` &limit=`10`]] 

The limit property is necessary. If you want to show all images, set the limit to a really high number, as setting the limit to 0 does not work with an offset.

Are you also using pagination? In that case, the offset approach will not work as expected, as getPage will overwrite the offset and limit properties depending on the page number. An alternative approach in that case is to filter out the first image in the gallery with a &where condition, like this:

[[!mgGetImages? &where=`{"sortorder:!=":"1"}`]] 

The downside of this approach is that it may not work as expected if the image with sortorder 1 is marked as hidden/inactive.