This help article has a section on importing your own custom shapes (about half way down) and which folder to put them in:
let me know if that article doesn't have all the information you need.
Hey PAVAN B,
To set a shape as a default shape for a dimension you may do the following:
- Right click on your dimension
- Select 'Default Properties'
- Select 'Shape'
- Set your desired shape as default
For Dimensions you may also set default Colours, Sort and Comments. For Measures you may set default Number Format, Aggregation and Total Using in addition to Colour Palette and Comment
Hope this helps