Any program should let you do that really. They may not give a pretty way to use a dynamic pool of hashtags, but you can do it manually with spintax which would work.
Other things to consider would be, do you want to dynamically tag users in the image, dynamically mention, post first comment, set locations, add watermark, auto update caption per x time period.
ahh just read you want cloud based, that does limit you somewhat. Unless you would consider running desktop apps in a VPS which gives you many more options
I just like cloud based softwares because they have their own proxies and they are good.
Any desktop app can be run on a VPS, so if you find one that suits your needs, you could do that if you don't want to run it locally. As far as I know there are more desktop app options and they are much much more feature rich than any cloud based solution I have seen which tend to offer only basic functionality (which may be totally fine for what you need)"Unless you would consider running desktop apps in a VPS which gives you many more options."
which software did you have in mind
which softwares do you know that have dynamic hashtags
Just to clarify, by dynamic tags, you mean you dump in a list of say 100 tags, then specify you want 3 tags in each image that is posted at random?