It sounds like to me you have set a restriction on the bandwidth (or to be more precise the data allowance) you are using.
For example if your on a pay as you go 3g dongle you may not wish to download anything more than 64k in size, so it will download the first 64k of the picture and render that in the space expected for the whole graphic.
It is not unusual to do this if you are paying a high premium for the data your using - have you switched bandwidth saving or a similar setting on.
Usually in these situations you can right click on the graphic and select download full - if it is something of interest.
Its normally there to prevent you running up a huge data bill.