there could be a lot of factors

like your pictures on one page appearing one size when they are actually another size

example of that

one picture says it is 198x134 when it is actually 768x432

big difference in that since they loaded slow.

you have to work with the template you chose so may be limited to what you can have in the space area of the columns and huge text or pictures that go beyond the column space could make it look off and more things

Be sure that you are using the height and width attributes in the tag. If it is set by default, change them to the original size. This could be due to optimization. These attributes are also optional, so if you don't put them, it won't do anything.
I hope this helps!

Always resize your images before uploading. You probably already have some software that can do the job on your computer, if not, download Irfanview.

Also, never use .bmp images on the internet (you have one on the downloads page)