Based on feedback from the submission portal's technical support, this is likely caused by multiple layers in the EPS/PDF file that has been uploaded. (This may happen with TIFF and PNG files as well.)
Such issues can usually be avoided by flattening the EPS/PDF files, before submitting them along with the manuscript file. This will merges all layers of the image into a single layer with no alpha channel.
Photoshop and GIMP, as well as some other image manipulation programs, contain such "Flatten image" or "Merge layer" functions. You can also use command line commands, such as ImageMagick, or use
pdftops to do a round-trip conversion. For example, for a PDF image called "fig1.pdf", run:
pdftops -eps fig1.pdf
to get an .eps, and then
to get back a flattened .pdf.
If you still see issues with your images after trying these steps, please contact us and we'll see if we can help.