I don't think you're going to have much success with that as they are meant to be durable and hold for a long time. If the image was applied with ink (like silk-screen) then it will be attached to, and somewhat into, the fibers of the cloth. Pretty much the same thing if the image is applied from a transfer sheet as it is pressed onto the fabric with heat and pressure. Depending on how well it was applied, and by what technique, it may be near impossible to remove it, but if it wasn't done very well.... The best option I can think of would be to rub the surface of the image together, against itself, to break down the image surface and make it fall apart without too much damage to the fabric itself.
Best of luck.
Here is a measurement chart that includes Delf (listed under Luts) and DD10 and SD13 (listed under Volks)
Note - the chart may be a bit old so compare the chart's Delf measurements to those on the eLuts.com site Delf sales pages to make sure they are current (looked close to me).
And this has various size BJD measurements
And you will find a bigger list of measurements where each of the BJDs are sold (things like leg length and shoulder width etc)
You can compare Delf to SD13 measurements.
Looks to me like Delf F is smaller in chest and waist and larger in hip than SD13 F ?
In general Volks BJDs are chunky and some models are immature. That's because SD10 was supposed to represent a 10 year old child and SD13 a 13 year old child.
I usually tell people to avoid clothes labeled "SD..." (for 1/3 BJDs and "MSD" for 1/4 BJDs of they are trying to buy clothing for a slim mature BJD.
Basically some SD13 stuff may work for your Delf and some won't, just like with any other BJD's clothing. Stretch clothing obviously can stretch a bit more. Tight non-stretch skirt or slacks made for a BJD with a smaller waist or hips will not work on larger waist or hips. Etc. Clothing that is large in some dimensions can sometimes be adjusted by moving the back closure or taking in the sides a bit along a seam.
You may do better searching Ebay for Delf, then select the doll category, order by price low to high and look what clothing sellers are selling for Delf. You can also search for 1/3 BJD clothes and compare your BJD's measurements to the measurement of the BJD that the clothing was made for or the BJD that is modeling it (you can ask the seller that).
And you can compare your Delf's measurements with the measurements of other similar height 1/3 female BJDs - if she is close to another popular BJD maker's dimensions (even for just tops of bottoms) then you can search for that BJD's clothing.
Delf females were one of the most popular 1/3 size BJDs that people sewed a few years ago - they sewed for SD models, Delf and Rainy (I think Elfdoll Rainy BJDs may no longer be made). Those were the 3 sizes I would see offered in handsewn 1/3 BJD clothes. Now there are a LOT more BJD makers and owners aren;t clumped as much in the most popular brands.