So, I have been spending the last several hours, uploading my files. In my computer, I have been using pdfLaTex and my images are in .pdf format.
Long story short, to trigger Elsevier's server/computer to compile using pdflatex.exe instead of latex.exe, I found out that adding this into the first line might help:
%&pdflatex
Trying it now..
Results: it worked!