InVision will let you do this, I think:
Design In Your Tool of Choice
InVision simply requires .jpg, .png or .gif files. Create them however you want.
Drag 'n' Drop
Take your static files and drop them right into InVision all at once. The bulk uploader makes adding files a snap.
Draw Hotspots & Link 'em up
Linking screens together couldn't be easier. Simply draw a hotspot and select a target screen for that button to click through to.
It looks like you upload images and then use their tool to define which parts of each image link to which other image.
However, it's a design tool, not a testing tool, so you probably can't do remote usability testing easily. You can however share a link to the wireframes so people can look through it and you could do usability testing by getting participants at the office to sit down and click around.