I’m sorry that the Omni people haven’t responded, but as a fellow user who encounters so many issues with their software I cannot reproduce this problem. I have tried pasting, placing, etc with png, jpg and pdfs and they all show up fine. I wonder what might be wrong.
As a UX researcher for 20 years I still cannot fathom how OG and other tools aren’t researched well but I hope you can find out how that happens.
In general, if you’re encountering an issue with our software I recommend that you reach out to our support team directly using Help > Contact Omni (which for this app would send email to omnigraffle@omnigroup.com). When you reach out directly, our support team typically responds within one business day.
As for this specific question, this appears to be an operating system bug where the image composite operation fails because the system is having trouble allocating more video memory. If you open Activity Monitor when this happens, and switch to the Memory tab, it would be useful to know how much memory pressure you’re seeing when you look at the graph and stats at the bottom.
Restarting OmniGraffle might clear this up, if the memory pressure is caused by OmniGraffle itself. If the memory pressure is caused by other processes in the system, a system reboot might clear it up.
You can also try changing the way OmniGraffle draws its images, so it uses different API that is slower but less likely to run into this issue. To do that, you would run this command in Terminal: