Touring the Interface with a Basic Composite - Nuke 101: Professional Compositing and Visual Effects - page 70

7. In Write1's Properties panel, click the Read File check box to turn it on.
YourWritenodeshouldnowlookliketheonein FIGURE 2.39 (nextpage).Itnow
has a thumbnail like a Read node does.
Figure 2.39. The Write node now has a thumbnail.