I get black lines above and below graphics and pictures that I have reduced in size in RED. My normal routine is to import oversized graphics into Photoshop Elements 8, resize, save with transparency, export to HD and then import into M100. If I resize the graphics in RED, say to start smaller and bring to 100% or greater, I get a black line above and below the graphic or picture.
Is this indicative of an alpha channel problem or something else?
Usually when this happens to me, it is something I missed when I created the alpha channel. Stray lines along the border edge or stray pixels in the alpha channel that aren't 100% black or 100% white.
You can fix it in the image editor of your choice. Sometimes you can tweak things in RED with a matte choker, or by cropping the edges a bit, or creating a new mask to address the garbage areas.
Another thing I noted is in relation to borders on graphics with alpha channels. If your graphic element is cropped tightly, then you apply a border in RED (too thick or too soft), it will sometimes crop that border if it exceeds the boundary edges of the graphic. I might drop the graphic into a composition clip and apply the border there in this case. Or give the original graphic some more buffer area around its edges.