If you have ECW map images without world files, they may work fine in ArcGIS but MapInfo won't put them in the right place as it can't read the header information in the ECW. You could opening them individually in MapImagery to generate tabs, but this is very time consuming.
We have written a custom MapBasic app for this purpose: for ESDM staff this is located in 3rdPartyApps\TABs_from_ECW [this being a pretty dumb place for it, as it is not a 3rdPartyApp, Ed]. Instructions are in the text file in the zip.
If you are not exeGesIS staff, and find yourself in this situation, please contact us for help.