I looked at the documentation from Jquery.Lazyload and found the following:
When Images Are Not Sequential After scrolling page plugin loops though unloaded images. Loop checks if image has become visible. By default loop is stopped when first image outside viewport is found. This is based on following assumption. Order of images on page is same as order of images in HTML code. With some layouts assumption this might be wrong. You can control loading behaviour with
failure_limit
setting.$("img.lazy").lazyload({ failure_limit : 10 });
Setting
failure_limit
to 10 causes plugin to stop searching for images to load after finding 10 images below the fold. If you have a funky layout set this number to something high. Worst case being the actual number of images.
Source: http://www.appelsiini.net/projects/lazyload
I therefor assume that your sidebar is defined in the bottom of your html page and therefor not loaded until you scroll down.
Small update:
In the documentation is the following tip: since you do not define a width/height in either html or css it might not work properly in all browsers.
PRO TIP! You must set image dimensions either as width and height attributes or in CSS. Otherwise plugin might not work properly.