It's hard to tell from their homepage that CARAT -- the Catalog of Arkansas Resources and Treasures -- is a listing of their online databases. This is the place to head first for digital content. You can group-search several data sets at once, including:
Other stopping off points are:
Arkansas Photo Collection (online images are low resolution, which seems a shame..hi rez copies can be purchased.)
Black History Commission of Arkansas includes mostly pointers to offline materials.
Arkansas Civil War site, with a wide variety of pages on the war itself, and life in antebellum Arkansas.
There is also a (somewhat hidden) Documenting Arkansas page with a small but significant collection of images, maps, historical documents, and so on...wish they didn't use the very unfriendly CONTENTdm platform, though.
At the Arkansas History Commission there is another search interface for combing through archival content. Use the Power Search features to pinpoint what you're after. For instance, setting Type to Photographs will access a large collection of digital images.