diff --git a/aniphallow.lua b/aniphallow.lua
index a12a442..d3e9050 100644
--- a/aniphallow.lua
+++ b/aniphallow.lua
@@ -1616,6 +1616,8 @@ openPreviewWindow = function()
             local currentFile = app.sprite and app.sprite.filename or ""
             if currentFile ~= lastSpriteFilename then
                 lastSpriteFilename = currentFile
+                -- Always refresh source when switching files
+                refreshSource()
                 -- Look up preset for this file
                 local presetForFile = filePresetMap[currentFile] or "Default"
                 if presetForFile ~= S.currentPreset then
@@ -1641,6 +1643,8 @@ openPreviewWindow = function()
                         openMainDialog()
                     end
                 end
+                -- Always repaint preview on file change (even if preset is same)
+                pcall(function() previewDlg:repaint() end)
             end
         end
     }
@@ -2656,13 +2660,25 @@ openMainDialog = function()
         text = "Analyze",
         visible = false,
         onclick = function()
-            if not S.sourceImage then
-                refreshSource()
-            end
+            -- Always refresh to avoid stale image from a different sprite
+            refreshSource()
             if not S.sourceImage then
                 app.alert("No source image. Open a sprite first.")
                 return
             end
+            -- Diagnostics
+            local diag = "mode=" .. S.gbAnalyzeMode ..
+                " tileW=" .. S.gbTileW .. " tileH=" .. S.gbTileH ..
+                " imgW=" .. tostring(S.sourceImage.width) ..
+                " imgH=" .. tostring(S.sourceImage.height)
+            if S.sourceImage.width == 0 or S.sourceImage.height == 0 then
+                app.alert("Source image has zero size. " .. diag)
+                return
+            end
+            if S.gbTileW < 1 or S.gbTileH < 1 then
+                app.alert("Bad tile size. " .. diag)
+                return
+            end
             local usedTileW = S.gbTileW
             local usedTileH = S.gbTileH
             if S.gbAnalyzeMode == "tile" then