Consider the following as deprecated. For updated info check the Image Galleries section in stash's app help/manual.

Stash offers support for image galleries. Here are some remarks on using them:

Galleries are zip-folders with images (e.g. jpeg or png) in them.
Stash searches for zip galleries in the same paths it searches for videos.
If you want to add a gallery to a scene, make sure that the zip file and the video file are in the same folder.
When storing images in zip file make sure to disable compression ( copy , store or no compression options depending on the software you use. Eg on linux: zip -0 -r gallery.zip foldertozip/ ). This impacts heavily on the zip read performance.
Stash uses the golang native ( pure go ) image decoders ( more suitable for cross compilation ). With huge images, decoding and converting to thumbnails can be slow and in cases cause visual errors or delays when loading the gallery page.
As of commit 9dacad7 stash adds a gallery to its related scene during the scanning process if they have matching names. Gallery /my/stash/collection/media_filename.zip will be auto assigned to /my/stash/collection/media_filename.mp4 (where mp4 can be avi, mkv, wmv ... or any other media file we support).
As of commit 29336d1 a cover image is supported. If an image called xxxxxxcover.jpg (where xxxxxx can be any filename) exists in the zip file it will be treated like a cover and presented first in the gallery view page and as a gallery cover in the gallery list view. If more than one images match the name the first one found in natural sort order is selected.
As of commit bd45daa the gallery thumbnails are cached. The first time you go to a gallery page the thumbnails of the images are created and stored to the disk cache for later use. If you want to populate the cache beforehand there is a new generate task that when ticked generates thumbnails for all the galleries during the generate task.
There is temporary CSS Tweak which enables a Grid View in gallery. Copy&Paste following CSS Snippet into Interface panel and enable custom CSS in settings:

.col.col-sm-6.mx-auto.table .d-none.d-sm-block {
    display: none !important;
}
.col.col-sm-6.mx-auto.table .w-100.w-sm-auto {
    width: 175px !important;
    background-color: rgba(0, 0, 0, .45);
    box-shadow: 0 0 2px rgba(0, 0, 0, .35);
}
.col.col-sm-6.mx-auto.table tr {
    display: inline-table;
}