Images hosted on other domain loading quicker??

The header image and sidebar images on my site are loading very slowly. The only exceptions are 2 images hosted on other domains (the 2 top ad squares). What doesn’t make sense to me is that all 4 ad squares are part of the same widget, but the 2 bottom ones hosted on my domain are taking as long as the rest of the sidebar images. (They are stored in my theme’s ‘image’ folder)

In addition, the images in my posts (stored in the uploads folder) are loading as soon as the page comes up, very quickly, while the header and sidebar images are the last to load.

One other piece of info – the header image was originally loading with a theme function call, but I’ve removed the function call in the header and am loading the header image with css. I wanted to see if it made a difference, but it hasn’t. ??

I am using the Panorama theme, the web address is https://www.polishedpicture.com. Thanks in advance for any help or advice!