White boxes appear around certain images when publishing online... i'm really not sure how to fix

In the advanced tab of the export dialog choose PNG as the format for graphics.

Open the images in Photoshop and see if they have white backgrounds. If they do, either remove the backgrounds and save as .psd, or use a clipping path. If they already have transparent backgrounds, check to see if the InDesign image frames have a fill color of white (called Paper in InDesign).