First I would make sure that RH2015 is completely up to date. Then I would zip up a copy of the project off the network & put it on my local c:\ drive in someplace with a short pathname. Then I would delete the .cpd file & try opening it again.
Also try in one of the sample projects.
If you open a different project and try to insert an image, does the same thing happen there?
If it does, it would seem that the issue is with RoboHelp or possibly with the image. To rule out the image, I'd try inserting an image from the RoboHelp Gallery.
Also, I'm very unclear as to why you feel you need images that are 300 DPI. Most images used for screen display are about 1/3 of that.
Check out this link for more on that.
I believe a 300 DPI image will be substantially larger than the normal screen image. So that may be a factor.
I did as you said and the project seems to be working fine on my local c:\ drive and with the .cpd deleted. I was able to open it and insert multiple images. What exactly does that mean for the original project though? Should I just continue working off the one on my local c:\ and delete the one on my network? Or do I need to do something to the network one. And yes, my RoboHelp 2015 is up to date.
With recent releases of RH Adobe has said that running projects off of network locations is ok, but everyone's network is different and issues may arise. That's why us "old-timers" stick with local copies of projects on "short" paths like c:\projects\project_name\. Of course all this changes when you have multiple authors at play - then you need to invest in source control so that topics can checked out/in when authors work on them. The .cpd file is just an Access file that keeps track of parts of your project - it's safe to delete it because RH recreates each time if it's missing.
I began using 300 DPI for scaling purposes. I use this so that when I shrink an image down in RoboHelp, the image doesn't become skewed. When I insert a 300 DPI image, I don't leave it in its original size but rather shrink it down to a size I feel is appropriate for the publishing the webhelp. If you think 300 dpi images, even when shrunk down, is causing the issue, I can try another route.
Thank you so much, Jeff. That answered my issue.