We seem to be having trouble with the definition of "fluid" interface design. Please tell me what this term (especially "fluid") means to you. Does it have a very specific and definite meaning? Does it demand certain image file types? Is it widely used terminology? Thanks!
It is just a buzz word.
They could mean they want the design to be flexible to different screen sizes (usually called Liquid Design), or they could mean they want a smooth user experience without a lot of interruption. You may want to ask for a clarification if they intended any specific requirements.
Perhaps these will help: