Yup, you'll have to edit the images to smaller size.
Read about image optimisation here
You can see that file size is determined by a variety of factors. In your case, decreasing the dimensions of the images will greatly help in reducing file size. You seem to be using massive images but actually resizing them down greatly in-game. This still uses the original image, however (and additionally has to calculate resizing it, which also looks pretty ugly in C2), so you're effectively wasting space.
Example, your iodine bottle:
Original, actual image [703x1770px; 47KB]
How the image appears in-game [61x153px; still 47KB]
Resized image to fit in-game [61x153px; 5KB]