This is probably the most commonly asked question, and it's an important one. At the moment we only offer web annotation and no desktop editing client as you may be familiar with other solutions in this space. We understand this is a different paradigm that takes some getting used to. We have made this decision for several reasons.
First, it's much easier and faster for us to make improvements to our tool on web. Second, it's more universal meaning it works on Mac, Windows and many other OS as well as mobile web (which almost no other annotation tools work on).
Additionally, web based gives us more opportunity to integrate with other tools you use already. For example, our integrations with Slack and Google Drive enable users to "open with" so files can be annotated directly from those tools. This would not be possible with a desktop editor. Another benefit includes the ability to share files across devices and locations. So basically you can view history and edit anything anywhere. The downside to this approach is that it can be a bit slower because of the upload/render factor vs. just opening locally. But part of this is due to the way we build markups — that they don't get flattened after closing; allowing you to forever edit markups. Only a few other tools offer this capability. And, using web vs. desktop can be an issue for offline screenshots and annotations.
All that said, a desktop editor is something we are thinking about and may do in the future. We just don't want to be too hasty and be sure there is a real need here. We believe we can address a lot of the concerns above while still getting the inherent benefits of web.
We will always work to increase the speed for example. We also plan to allow screenshot only taking from desktop app that would copy the image to your clipboard. This could be used for offline screenshots and also for faster cases when you don't need to actually annotate, or annotate now.
When you get back online, or anytime later you can click to open in browser and annotate.
Please vote and comment on this idea on our public roadmap https://roadmap.markuphero.com/boards/feedback/posts/36