Hey Julia,
Ok. Upon further investigation it appears that image resizing in RTF (Rich Text Format) blocks has some limitations.
Good News - You can achieve the desired look you want
Bad News - It involves a little bit of work with manual tweaking.
Here are a couple of previous forum posts that tackle it:
Regarding optimising (compressing) images, yes, we try to make the image as small as possible in Kb before we upload to Webflow (once uploaded WF will do it’s thing to further optimise if needed)
They way we optimise images is to address image dimensions, then compression.
First, look at the dimensions of the image we need as to how it will look on the screen - no real benefit having an image 2400px wide if it will only ever need to be a maximum 800px wide on our site - so we resize the image to 800px wide (keeping aspect ratio). This will immediately reduce the Kb size of the image file.
This free online tool is an example:
Second, reduce image file size further by compression, removing unneeded extra info within the file (info like, date picture taken, GPS co-ordinates etc)
This free online tool very common:
https://tinypng.com/
Let us know how you get on,
Keiran