Images not showing in Theme ????

I just started building a template for a site and have run into a brick wall. Let me explain;

I put together some of the graphics for the header.php file and uploaded the images and the file to my server, everything was fine and all images showed up.

I decided to change a few of the images and uploaded all files, now all the images that I changed and any further images that I add do not show, unless I use an absolute URL (https://xxxx.com/wp-content/themes/mytheme/images/xxx.gif)

Where as images/xxx.gif should work.

(“images/xxxx.gif”) works from the css file, but any images added to the header.php do not, that is any images changed or added after the second upload.

Thank you, Jer