When writing a post you can upload images. There is a size limit of 500kb on each uploaded image - do not try to upload any larger images as it simply will not work. If you need to show images larger than 500kb you will need to Link to them elsewhere on the web (e.g., Flickr) - but as you will see from below it is unlikely that will be required with a little bit of work with Photoshop or some other image program.
Usually, images from todays digital cameras are much bigger than 500kb, typically they are several MB! I recommend you use one of the many photo manipulation packages available to reduce the dimensions (ie pixel size) which will of course reduce the size of the image, but more importantly pictures are best saved in JPG format and this format allows a number of quality settings (%) - typically for example using Photoshop you can save with medium-high (30%-60%) and get a much reduced file size without compromising the on screen viewing pleasure (note do not use these on screen optimized pictures for you photographic printing - always use the originals for best quality)
Below is the same picture in various modes for you to directly compare:
The Original Image was 1536×2048 pixel and approx 1.6Mb in size! Far to big for this Blog!
I then scaled this image down to a more manageable 600pixels wide (800 tall as it happens) result in a file size of 384k!

The next image was saved for web in photoshop using Maximum (100%) setting resulting in a file size of 348kb:

Next image is at Photoshop Very High (80%) setting and has a file size of 124kb!

The next has a photoshop setting of high (60%) only 62kb in size now!

Now we are down to a medium (30%) setting on Photoshop - file size of 28kb! (check the sword and helmet areas for corrupted pixels)

Finally the Low (10%) setting in photoshop for a tiny file size of 18kb!

As you can see there is very little difference between some of the images, typically the lower quality setting you use then expect some pixels deformation around edges of objects… I would recommend you stick between 30 %and 60% if using photoshop (or PS elements etc) - but you have to judge what best suits your images as some may require a higher setting than others.
For non photographic images the GIF is generally best - i.e., for images of flat colours like diagrams.
Hope this has been useful.