> Is there any way to tell LyX that the pdf-file exists and needs to be > copied to the temp-folder, so latex can find it?
Martin, I had a similar problem and have to thank the latest version LyX 2.0.1 for solving the issue. This problem occurs when your figure (both .pdf_tex and .pdf) in a directory (most commonly a subdirectory). To cut it short, let us assume that the two files are in a subdirectory ./figures (I would recommend to always use one such name for the directory for the reason to follow). Go to Tools->Preferences->Paths->TEXTINPUTS prefix: by default the current document directory is included (the dot .). Add the subdirectory path by appending ;./figures (the semicolon ; is to separate the two paths). I believe this should solve such issues in he future except in the case that your subdirectoery has a different name (e.g. ./img) in which case also append ;./img. Regards, Mukhtar