How to resize  element within drupal block?
Is that everything that's in your block? I've tried to replicate this and it works fine for me.

Have you tried setting your CSS in the appropriate css file instead of adding it inline? Like: 

#block-id-title img {
   height: 100px; 
   width: 100px;
}


Or, try inspecting that img element in firebug or equivalent, and see how the css is being rendered. You might have a css styling set for all img tags which is unintentionally overriding your block image.