iMovies goal is "projection" on TV - in PAL countries that has a screen resolution of 768x576 (.. and that is max, in real life it is less...)
second, a computer has no interlacing - that is a good trick for moving images on TV, but that "blurrs" any detailed content...
sooo....
you charts will be minimized & blurred. in case you used any serif-typo, the tiny lines will vanish/flirr.. that isn't iM fault, but due to TV standards....
I would:
use Excel to export directly into some pict format (tiff isn't bad, because no compression); I don't see any need to make an inbetween step in some slideshow app ... (???)
and I would "tune" in Excel the formats: not too much content, san-serif typo, no bright colors (as pure red), no 1pix lines etc....
best practise:
re-create new, "video compatible" charts in some pic maker (e.g. Photoshop) by using the correct settings from the beginning - you instantly see what you get...
don't forget: judge pic quality of iM projects only TV screen!
______
besides:
dpi is an internal-file-info just need for PRINTS... a 10x10pix picture is displayed as 10x10 pix on every computer's screen... independed of 72dpi, 120dpi, or 2400dpi... bigger dpi's make file size larger, but not better for monitor use.... 😉