When you copy from your plotting application, you seem to be copying vector data, possibly postscript or maybe something else if it's old like pict file. When it gets to KN it is not interpreting it correctly and somehow generating an error that doesn't let you resize. Vector art for a plot has advantages over a bit map file in size and in not being resolution limited in the way a bit map is. You could try printing a PDF from your plotting application to keep the vector art. You may need to mask the result in keynote to hide bit's of printout you don't want. If you have a vector-based drawing application you can edit the file itself.
To make an image a placeholder select the image and use *Format Menu> Advanced> Define as Media Placeholder*. This allows you to hot-swap the file being used so that the placement, opacity and any builds you assigned to the previous image stay applied to the replacement image. It should not make any difference to your ability to resize the image.
MG1234 wrote:
Anyway thanks again, now I can stop using powerpoint totally.
yeah, that's a huge relief